SPF/DB hollow core fan blade

ABSTRACT

A hollow core fan blade for a gas turbine engine, having a continuous leading edge, is fabricated using a four-sheet superplastic forming/diffusion bonding process which results in a cost-efficient and lightweight, yet strong, structure. The rotor blade is comprised of a face sheet which has a 180 degree bend therein so that the two face sheet ends are aligned. Bonded to opposing sides of the face sheet are first and second core sheets, between which is the hollow core. To fabricate the blade, a core sheet assembly is inserted inside the prepared face sheet, thereby forming a Ti-Pack (titanium pack) assembly having a plurality of pressure-tight cells. The Ti-Pack is inserted into a cavity within a die, after which the rotor blade, having predetermined design characteristics, is superplastically formed by heating the die and selectively pressurizing the plurality of cells.

BACKGROUND OF THE INVENTION

This invention relates to the production of hollow-core structures, andmore particularly to the production of a superplasticallyformed/diffusion bonded hollow core rotor blade for a gas turbineengine, especially a fan blade, and the procedure for producing such ablade.

Superplasticity is the characteristic demonstrated by certain metals todevelop unusually high tensile elongations with minimum necking whendeformed within a limited temperature and strain rate range. Thischaracteristic, peculiar to certain metal and metal alloys, has beenknown in the art as applied to the production of complex shapes. It isfurther known that at these same superplastic forming temperatures thesame materials can be diffusion bonded with the application of pressureat contacting surfaces. One particularly well known process forproducing superplastically formed structures, known as the "four sheetprocess", is described in U.S. Pat. No. 4,217,397, assigned to theMcDonnell Douglas Corporation, and herein incorporated by reference.

In a continuing effort to improve gas turbine engine operatingefficiencies, as well as to permit the development of transport aircrafthaving greater passenger and cargo capacities, engine manufactures havebeen designing increasingly larger engines. These new generationengines, known as "high-bypass engines" or "very high bypass engines",typically operate with a bypass ratio approaching or exceeding 80%,meaning that 80% or more of the total airflow into the engine bypassesthe core engine (consisting of the compressor, combustor, and at leastthe high pressure turbine) and instead flows only through thesurrounding fan section, which includes the fan blades and perhaps thelow pressure turbine. In a high-bypass engine, most of the generatedthrust is derived from the bypass air, enabling higher fuel efficiencyand the lower noise output necessary to meet increasingly stringentnoise regulations. As a result, engine fan diameters continue toincrease in size, and it becomes ever more critical to reduce thestructural weight and dynamic loading in the rotating portions of theengine. Estimates are that an effective hollow core fan blade design fora typical large high bypass engine would reduce engine weight by 150pounds, which would in turn reduce specific fuel consumption by about5%. As future engine fan diameters increase, it becomes even morecritical to reduce the structural weight and dynamic loading in therotating portions of the engine.

Current fan blade configurations are fabricated from solid titaniummaterials. This is due to manufacturing cost considerations, as opposedto structural load requirements. Therefore, if a cost effective titaniumhollow core fan blade could be manufactured, it would be able to meetthe structural load criteria for safe operation. Dynamic loads withinthe engine core would also be reduced with the reduction in blade mass,which would in turn allow the entire engine core to be furtheroptimized. Future engine growth would occur without requiring the costlyredesign of core sections.

It is known in the prior art to manufacture hollow core fan blades forlarge gas turbine engines by machining matching cavities in titaniumplates, then diffusion bonding a honeycomb core inside the cavity.However, this is an extremely expensive process and the resulting bladetends to be vulnerable to damage, in part because it has a discontinuousleading edge. What is needed, therefore, is a hollow core fan bladewhich may be manufactured by a cost efficient superplasticforming/diffusion bonding (SPF/DB) process, and which is more damagetolerant than currently known hollow core blades.

SUMMARY OF THE INVENTION

This invention solves the problem outlined above by providing a hollowcore rotor blade for a turbine engine which may be relatively easilymanufactured by an inventive and cost efficient SPF/DB process. Thehollow core rotor blade comprises a generally airfoilshaped outerstructure comprised of a superplastically formable, diffusion bondablematerial. The outer structure has a trailing edge and a leading edge,and encloses a hollow core spacing. A key feature of the rotor blade isthat the leading edge is continuous and seamless, thereby allowing therotor blade to be relatively lightweight, aerodynamically efficient, anddurable.

At least one and preferably a plurality of structural webs extend withinthe hollow core spacing of the blade. One advantage of the inventiveprocess is that the webs may be oriented either vertically orhorizontally, or both. The rotor blade is preferably a fan blade for agas turbine engine, although other applications are possible.

Another major advantage of the invention is that the leading edge iscontinuous, since the face sheet outer surface also comprises the outersurface of the rotor blade, with the leading edge being locatedsubstantially at the point of the 180 degree bend on the face sheet.

In another aspect of the invention, a rotor blade assembly for a turbineengine is disclosed, which includes the above-described rotor blade aswell as a root section, with the rotor blade being mounted on the rootsection. Preferably, the root section is a hollow, flared out portion ofthe blade itself, with the hollow portion being filed with a solidmaterial filler.

In yet another aspect of the invention, a method of fabricating a rotorblade for a turbine engine comprises the steps of preparing a core sheetassembly and preparing a face sheet having a first edge and a secondedge. The face sheet is bent approximately 180 degrees at itscenterline, such that the first and second edges are aligned. Then, thecore sheet assembly is inserted inside the prepared face sheet, therebyforming a Ti-Pack (Titanium Pack) assembly having a plurality ofpressure-tight cells. The Ti-Pack is inserted into a cavity within adie, after which the rotor blade, having predetermined designcharacteristics, is superplastically formed by heating the die andselectively pressurizing the plurality of cells.

The above mentioned and other objects and features of this invention andthe manner of attaining them will become apparent, and the inventionitself will be best understood, by reference to the followingdescription taken in conjunction with the accompanying illustrativedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-4 are perspective views showing sequentially the steps forpreparing the core sheets which form in part the Titanium-Pack (Ti-Pack)of the invention;

FIGS. 5 and 6 are perspective views showing sequentially the steps forpreparing the face sheet which forms in part the Ti-Pack of theinvention;

FIG. 7 is a perspective view showing the assembly of the Ti-Pack of theinvention wherein the core sheets are slid into the face sheet;

FIG. 8 is a cross-sectional view along lines 8--8 of FIG. 7, showingdetails of the gas inlet arrangement into the face sheet and the outercore sheets of the Ti-Pack;

FIG. 9 is a cross-sectional view along lines 9--9 of FIG. 7, showingdetails of the gas inlet arrangement to the inner core sheets of theTi-Pack;

FIG. 10 is a cross-sectional elevational view showing the assembledTi-Pack within the forming die;

FIG. 11 is a cross-sectional elevational view similar to FIG. 10 showingthe gas management system for ensuring proper gas flow into each of thecells of the Ti-Pack;

FIG. 12 is a diagrammatic view showing the pressurized cell geometry fora typical cell during the superplastic forming process of the invention;

FIG. 13 is a cross-sectional elevational view similar to FIG. 10 showingthe formation of the face sheet during the superplastic forming processof the invention;

FIG. 14 is a diagrammatic view showing the core sheet force balanceduring the blade formation process;

FIG. 15 is a cross-sectional elevational view similar to FIG. 10 showingthe formation of the outer core sheet during the blade formationprocess;

FIG. 16 is a cross-sectional elevational view similar to FIG. 10 showingthe initiation of the inner core sheet during the blade formationprocess;

FIG. 17 is a cross-sectional elevational view similar to FIG. 10 showingthe inner core sheet formation;

FIG. 18 is a cross-sectional elevational view similar to FIG. 10 showingthe blade positioned within the die after diffusion bonding is complete;

FIG. 19 is a cross-section of the trimmed fan blade as finally formed bythe process of the invention;

FIG. 20 is a perspective view of the fan blade assembly of the inventionafter the fan blade has been attached to the machined root section.

FIG. 21 is a cross-sectional elevational view similar to FIG. 10,showing an alternative embodiment;

FIG. 22 is a cross-sectional elevational view similar to FIG. 13 showingthe embodiment of FIG. 21 after formation of the fact sheet;

FIG. 23 is a cross-sectional elevational view similar to FIG. 16,showing the embodiment of FIG. 21 after initiation of the inner coresheet during the blade formation process;

FIG. 24 is a cross-sectional view similar to FIG. 19 of the trimmed fanblade as formed in the alternative embodiment of FIG. 21;

FIG. 25 is a perspective view similar to FIG. 20 of an alternativeembodiment fan blade assembly; and

FIG. 26 is a cross-sectional view taken along lines 26--26 of FIG. 25showing details of the blade root section.

DETAILED DESCRIPTION OF THE INVENTION

As indicated above, the material to be superplastically formed mustexhibit the characteristics of unusually high tensile elongation withminimum necking when deformed within a limited temperature and strainrate range. While several materials demonstrate these superplasticproperties, titanium and its alloys are currently the best known formingmaterials. The superplastic temperature range varies with the specificalloy used, however, the temperature just below the phase transformationtemperature is near optimum. This temperature for titanium alloys isnear 1700° F. The best strain rate is actually determined experimentallyfor each configuration formed, in order to produce balanced deformation.If the strain rate is too rapid, it will cause blowout of the materialbeing deformed, and if the rate is too slow, the material loses some ofits plasticity.

In addition to the superplastic properties, the material to be formedmust be suitable for diffusion bonding. Diffusion bonding, as usedherein, refers to the solid-state joining of the surfaces of similar ordissimilar metals by applying heat and pressure for a time duration tocause co-mingling of the atoms at the joint interface.

The inventive process for manufacturing the hollow core fan blade of theinvention, disclosed herein, is a variant of the four-sheet processdisclosed in U.S. Pat. No. 4,217,397, herein incorporated by reference,as noted above. The first step is to fabricate a Titanium-Pack, or"Ti-Pack" 10, for insertion into a die 12 (see FIG. 10). Then, the nextstep is to initiate the SPF/DB process, so that the Ti-Pack 10 will beformed into a fan blade having the desired characteristics.

Ti-Pack Fabrication

The Ti-Pack 10 comprises all of the structural members in their sheetmetal form, and a series of gas inlet tubes that will be used to supplythe gas pressure necessary for the subsequent hot forming operation.Referring to FIGS. 1-4, Ti-Pack assembly is initiated by preparing thecore sheets which form a part of the Ti-Pack. Two sheets 14, 16, whichin the preferred embodiment comprise 0.040 inch thick sheets ofTi-6Al-4V material, are trimmed to the proper perimeter dimensions forensuring a resultant fan blade of the desired size. Core sheet 14 hasfirst and second ends 14a and 14b, respectively, as well as first andsecond surfaces 14c and 14d, respectively. Similarly, core sheet 16 hasends 16a and 16b as well as surfaces 16c and 16d. One end portion 18, 20of each sheet 14, 16, respectively, is then bent approximately 90° toform an L shaped cross-section, as shown in FIG. 2. The sheets 14, 16are then cleaned and placed back to back for a roll seam weldingoperation. A rectangular grid pattern of intermittent spotwelds 22 isrolled onto the sheets to attach them together, although of course agrid pattern of any desired shape could be employed. The location ofthese welds dictates the final internal web geometry of the bladestructure, as will be explained in further detail hereinbelow. After theintermittent spotwelds are completed, the perimeters of the core sheetsare continuously spotwelded, except at locations where gas inlets are tobe installed, to provide a pressure tight seal that will contain thecore gases during forming. Following this, the core sheets 14, 16 arefolded back and tack welded at points 24 and 26 to hold them in place,as shown in FIG. 4. At this point, each core sheet 14, 16 comprises bothan inner core sheet section 28 and an outer core sheet section 30. Theinner core sheet sections 28 each have an outer surface 28a and an innersurface 28b, while the outer core sheet sections 30 also each have anouter surface 30a and an inner surface 30b.

Gas inlets 32, 34, 36, 38, and 40 are now installed in the core sheets,as particularly shown in FIGS. 8, 9, and 11. One inlet is required foreach of the cells 42, 44, 46, 48, and 50 formed by the grid pattern ofspotwelds 22 within the core sheets 14 and 16. Actually, inlets 32 and36 comprise a single inlet, since they are both directly connected topressure regulator P4, and cells 44 and 46 will function as a singlecell since they will be at identical pressures. Shown particularly inFIGS. 8 and 9, for each inlet a small titanium fitting 51 is used tospread the core sheets 14, 16 apart to provide a gas passage. Onto thisfitting a titanium tube (not shown) is fusion welded to connect theTi-Pack 10 and a gas source 52. A steel tube 53 is then used as a collarto prevent the titanium tube from expanding under the gas pressureduring the hot forming operation.

Once the edges of the area around the titanium fitting and the coresheets 14, 16 are sealed by fusion welding the sheets together, the coresheets 14, 16 are leak checked to insure that they are pressure tight.It is critical to find and repair any leaks, since leaks present in thecore will prevent the core from maintaining a positive pressure and willresult in part failures.

Assembly of the core sheets 14, 16 is essentially complete at thisstage, and preparation of the face sheet 54, shown in FIGS. 5 and 6, nowmay begin. In the preferred embodiment, the face sheet is a 0.050 inchthick sheet of Ti-6Al-4V, which is trimmed to the proper dimensions andmechanically bent at the sheet centerline 180 degrees so that the edges54a and 54b of the sheet opposite of the bent radius line are aligned,as shown in FIG. 6. The face sheet 54 has a first, or inner, surface 54cand a second, or outer surface 54d. Due to the material springbackeffects the edges will have a tendency to open, but this is not aproblem. Once formed, the face sheet 54 should be etched to provide aclean surface for bonding. The core sheet assembly 56 may now be placedinside the face sheet 54, as shown in FIG. 7, so that the trailing edgesof all the sheets line up. Once assembled, the first core sheet 14 isbonded to a first portion 54e of the face sheet inner surface 54c, whilethe second core sheet 16 is bonded to a second portion 54f of the facesheet inner surface. Face sheet inner surfaces 54e and 54f oppose oneanother. The periphery of the assembled Ti-Pack 10 is then welded closedin order to provide a sealed bladder that contains the sealed core sheetassembly 56 within it. To accomplish this, the edges of the Ti-Packassembly 10 are fusion welded together except at each of the gas inletlocations for the core sheet and the face sheet, the fusion welded edge58 being shown in FIG. 8. Gas inlets 60 (see FIG. 11) for the face sheet54 are installed in a manner similar to those of the core sheets 14, 16.Gaps around the gas inlets must also be carefully welded to seal theface sheet pressures. The entire Ti-Pack assembly 10 can now be leakchecked to verify the integrity of the welding. Once the Ti-Packassembly is pressure tight, it can be certified as completed and placedinto a steel die 12 for the hot forming operation (FIG. 10).

Hot Forming Operation

During the hot forming operation, the titanium sheets aresuperplastically formed into the final part geometry. This isaccomplished by placing the Ti-Pack 10 inside a cavity 62 within thesteel die 12, the die 12 then being mounted inside a heated platen press(not shown). By controlling the pressure, temperature and the relativetime at each of these variables the formation of the structure can bedictated by the weld patterns imposed on the Ti-Pack sheets. Formingtakes place by pressurizing the individual bladder systems, or cells,within the Ti-Pack 10. The forces generated on the titanium sheets causethem to expand and elongate until they fill the cavity 62, coming incontact with the tool surface. Flow stresses of the sheets arecontrolled as a function of the gas pressure feed to the gas inlets andby the temperature inputs to the tool. By following a mathematicallydetermined schedule for manipulating the gas pressure and dietemperature as a function of time, the internal geometry of the bladestructure can be controlled within the material's superplastic limits.

The gas management system for the inner core sheet sections 28, theouter core sheet sections 30, and the face sheet 54 is a rather complexnetwork. The configuration shown in FIG. 11 is derived from therequirement that each of the bladders, or cells, mentioned above mustform at different rates relative to one another. In this configuration,the gas source 52 supplies a gas (preferably welding grade argon gasbecause it is inert and will not contaminate the highly reactivetitanium at elevated temperatures) to a gas management unit 64. Gasmanagement unit 64 acts as a control unit for directing gas flow througheach of the pressure regulators P1, P2, P3, and P4 to the respectivecells 66, 48 and 50, 42, and 44 and 46. The gas pressure to the facesheet cell 66 is controlled by regulator P1, P2 controls pressure tocore cells 48 and 50, P3 controls pressure to core cell 42, andregulator P4 controls pressure to core cells 44 and 46. The gasmanagement unit 64 may comprise any known control means for selectivelydirecting gas flow to each of the pressure regulators on atime-dependent basis, in accordance with the control parameters setforth in detail below. The most important criteria is to maintain theequilibrium of forces within the Ti-Pack 10.

The fundamental equation that governs the forming rate for each of theindividual cells is simply the hoop stress equation modified to accountfor the transient nature of superplastic forming, where the cell radiusr and the material thickness t are constantly changing with respect totime. FIG. 12 shows a portion of a pressurized cell geometry for aninner core sheet section 28, depicting the pressure (P), radius (r), andthickness (t) parameters. The hoop stress equation is as follows:

    Hoop Stress=(Pressure*radius)/thickness                    (1)

This equation is valid until the cell touches the expanded face sheet,after which each side of the cell forms an individual corner radius.

The allowable dynamic stress is a function of the die temperature andthe strain rate sensitivity for the Ti-Pack material (Ti-6Al-4V in thepreferred embodiment) at its forming temperature. To calculate thegeometry changes, the stress variable is replaced with the strain ratemultiplied by the material modulus. The strain rate sensitivities formaterials are well documented in the technical literature and can becalculated for specific materials by performing a "Cone Forming" test toestablish this constant. Once the variables have been calculated, theequation can be rewritten as a function of the input variable, pressure,with respect to time:

    P(t)=(radius/thickness)*(modulus)*(strain constant)        (2)

The pressure input can now be plotted over time by calculating thechanges in sheet thickness and cell radii. These changes also dictatethe rate of forming within the internal cell geometry so that the degreeof forming can be predicted mathematically. By predicting the shape ofthe internal cells the new information can be continuously updated andinput back into the equations to develop the required forming parameterinputs.

The range that the strain rate must be kept is critical because if therate is either too fast or too slow the material will experienceexcessive thinning and a rupture will occur. Calculating the pressureinputs to stay within the superplastic strain rate of a given materialat the varying cell heights and material thicknesses is an empiricaltask, depending upon the specific material being utilized and thespecific finished configuration desired, and involves fairly extensive,though routine, computer modeling.

Having established the pressure schedule by mathematical means theTi-Pack 10 may now be loaded into the machined steel die 12. The die 12has the desired external blade geometry as its internal cavity 62. Thedie is then sprayed with a release agent such as Boron Nitrate whichfacilitates removal of the finished blade after forming. Each of the gasinlet lines are capable, by means of the gas management unit 64, ofadding or venting pressure between the core sheets and between the facesheet inlet lines. Once the part and die are securely loaded into thetool, the hydraulic pressure of the press must be adjusted to maintain apressure differential between the internal die cavity 62 and theexternal face of the die to keep the die closed. The input temperatureto the die 12 is increased from a loading temperature which may rangeabout 500° F., at a rate of about 3 degrees per minute, until the idealforming temperature (about 1650° for Ti-6Al-4V) is achieved

Typically, as the forming temperature is ramped up, the pressure in theface sheet cavity 66 is increased to initiate forming. The geometricsimplicity and lack of superplastic straining involved with pushing theface sheet 54 out to the die contour (the interior walls 68 of the diewhich define the cavity 62) allows the operator the option of initiatingthe forming at lower temperatures that have a narrower superplasticstrain rate sensitivity band. Although predicting the forming history ofthe face sheet is more difficult at lower nonconstant temperatures, thelack of overall material elongation increases the margin for successfulforming. FIG. 13 shows the face sheet 54 formed out to the die surface68.

As the face sheet 54 forms, pressure in the core sheets 14, 16 is heldat a constant value by the gas management unit 64 until the optimumtemperature is reached. Adequate pressure is held to keep the sheetsapart and prevent the inside surfaces from sticking. As the temperatureapproaches 1600° F. the pressure in the core cavities is increased.Wherever there is a row of spotwelds 22, a reaction point is providedfor the core sheets 14, 16 to wrap back around the weld nugget 70, asthe pressure in the core cavities increases, thereby developing a web orspar at that location. Thus, the location of these weld nuggets 70dictates the final internal web geometry of the blade structure. It isimportant to note that, as shown in FIG. 3, the spotwelds run in boththe vertical and horizontal direction. Thus, both vertical andhorizontal webs may be formed by the above described process, dependingupon whether a vertically or horizontally oriented weld nugget isinvolved, though only the formation of vertical webs is shown in thefigures depicting the fabrication process for the sake of simplicity.The relative pressure differential between each of the core cellcavities is a critical parameter that must be maintained so thatmovement of the web location does not occur. Unlike the typical foursheet core arrangement, the core sheets used to form the fan blade havemore than one pressure for the core sheets. This is required to balancethe core force system, as the reactions at the weld nuggets 70 tend topull the web in the direction of the smaller cell. Valves such as valve71 between pressure regulators P3 and P4 may be provided between the gasinput lines in order to permit equalized pressurization of all or someof the cells in the event that differential pressurization is notnecessary in a particular application.

Viewing FIG. 14, a typical cell geometry is shown. An unbalanced forcesystem in the X direction causes excessive material thinning whichresults in a blowout type failure. A balanced system is easy to achievefor the typical four sheet technique core arrangement because both edgesof the Ti-Pack 10 are restrained from movement by the clamping pressureof the dies. As long as the roll seam weld spacing is equal the radii ofthe cells remain equal and the webs have no difficulty forming.

Difficulties in forming the inventure blade are present due to theunique seamless continuous leading edge design of the blade. Thus, thedesign includes the pair of outer core sheet sections 30, which havebeen wrapped back around 180° to form an additional pressure cavity 48,50 within the Ti-Pack 10. This permits all of the edges of the sheets toprotrude out of only three sides of the forming die 12; at the bladetrailing edge, the top, and the bottom. The outer surface of the leadingedge being thus formed from a single sheet 54 of material, therefore isstructurally continuous, having no seam. By adding this feature, thecore sheets 14, 16 are no longer anchored at one end, therefore thebalanced force system in the X direction is upset. To counteract thisproblem, each cell must be individually pressurized at its own rate tomaintain equilibrium between the core sheet forces. Individual core cellpressurization also adds design flexibility by eliminating the criteriaimposed by the fabrication physics, that dictate each cell must have thesame weld spacing as the cell adjacent to it.

Like the face sheet 54, the outer core sheet section's simplicity allowsformation out to the face sheet at a fairly rapid rate. Once the outercore sheet section 30 has contacted the face sheet 54, as shown in FIG.15, diffusion bonding will occur and movement within the forming die 12will be restricted. As the leading edge side of the outer core sheetscontinues forming, the reaction vector for the internal core sheetsincreases as the forming angle flattens out to become parallel with theX axis. This reaction vector is held in place by the internal pressureof the outer core sheets and will provide a measurable force to reactwith one side of the inner core sheets at cell 42. The other side isreacted through cells 44 and 46 (best seen in FIG. 16) where that loadis finally transmitted into the clamped trailing edge 72. As thepressure is increased in cell 42, it will expand and the force vector atthe weld nugget 70 will decrease in the X direction and increase in theY direction. Concurrently while this is taking place, pressures in cells44 and 46 must be adjusted to balance the component forces in the Xdirection. As all three cells expand, as shown in FIG. 16, balancing theforces in the X direction becomes less important because the loads aredecreasing and therefore the movement of the webs is less prevalent atthis point in the forming cycle.

Again viewing FIG. 16, formation of the internal core geometry continuesuntil the inner core sheet sections 28 touch the outer core sheetsections 30 and the radius at the web/skin interface begins to close.Once the sheets come into intimate contact, the diffusion bonding cyclebegins to promote grain growth across the material interface. Underthese conditions of elevated temperature and pressure the intermetallictransfer of grain boundaries activates the solid state molecular bondingrequired for superior part strength and the elimination of stressconcentrations caused by joining methods.

FIG. 17 shows an advanced stage of forming where the core sheet webradii begin to gradually grow smaller, and the core pressure can beincrementally increased because the flow stresses in the material are afunction of the radius at any given pressure. In this stage, the innercore sheet sections 28 fold back over the weld nugget 70 and diffusionbond together. As the cell webs 74 become vertical and the corner radiusdecreases, the core pressure is elevated until a maximum of about 250psi is reached. This will be the pressure differential between the twomaterial interfaces that is required to diffusion bond the material andpromote grain growth across the boundary. After holding this pressurecondition and bumping the temperature up to about 1750° F. for two hoursthe diffusion bonding and forming portion of the hot forming operationare complete and the tool temperatures can be lowered.

The temperature at which the part is removed from the tool is a criticalparameter because it has a tremendous impact on the final dimensionalstability of the part. Due to the difference in thermal expansioncoefficients of the steel die and the titanium part, the die willcontract around the part as it cools. Since the part is still highlyformable at elevated temperatures a careful analysis must be done todetermine the overall machining factor used for the steel forming die.Typically the die is machined at a factor slightly less than 1.0.

The final part geometry is pictured in the steel die 12 in FIG. 18. Agradual cool down is required to maintain the thermal stability of thedie. As the part temperature approaches about 1400° F. the modulus ofthe titanium becomes adequate to permit removal of the blade 76 from thedie without damaging it. When the blade 76 is removed it should beplaced in an insulated container to eliminate any warping condition thatmay be caused by preferential cooling of an exposed surface. The bladewill now cool rapidly due to the large cooling area relative to itssmall mass. As the temperature nears 500° F., the gas inlet lines can bepinched off and welded closed. The hot forming operation is nowcomplete.

Cleaning and Trimming Operation

During the SPF/DB cycle the outer surface of the face sheet 54 isexposed to the atmospheric conditions present between the forming diesurface 68 and the Ti-Pack 10. Titanium is a highly reactive material attemperatures above 1200° F. This causes it to soak up impurities thatcontaminate the surface and degrade the mechanical properties. Toeliminate this contamination a chemical milling operation is used toremove 3 to 5 mils or so from the outer part surfaces. The "WhiteLayer", as it is commonly referred to, is not present within the webpassages of the finished component, because it only comes in contactwith the inert gas during the fabrication phase at elevatedtemperatures. After chemical milling a final machining operation isrequired to trim the component edges down to their net dimensions.

A cross-section of the completed blade 76 is shown in FIG. 19 with thetrailing edge 72 machined down to its final shape. Although theindividual lines of the titanium sheets are shown in the figure, all thesurfaces that contact one another are diffusion bonded together. Thus,the outer portion outer surface 30a for core sheet 14 is bonded to theface sheet first portion inner surface 54e, while the outer portioninner surface 30b is bonded to the inner portion outer surface 28a.Similarly, the outer portion outer surface 30a for the core sheet 16 isbonded to the face sheet second portion inner surface 54f. Each of thecore sheet and face sheet ends are bonded together to form the trailingedge 72 of the blade. Therefore, the component will essentially react tothe loading environment as a single piece of annealed titanium material.This single piece of annealed titanium material, comprised of thediffusion bonded face and core sheets, constitutes a generallyairfoil-shaped outer blade structure having a trailing edge 72 and acontinuous, seamless leading edge 88.

The final operation to complete the fan blade assembly 78, as shown inFIG. 20, requires attaching the hollow blade body 76 to the solidmachined root section 80. This is accomplished by fusion welding a butttype joint around the perimeter 82 where the two pieces come together.

Now referring to FIGS. 21-24, an alternative process for fabricating theinventive fan blade can be seen which is identical in all respects withthat of FIGS. 1-19 except as described and shown herein. Each of theelements in FIGS. 21-24 corresponding to equivalent elements in FIGS.1-19 are designated by the same reference numeral, preceded by thenumeral 1. Thus, a modified Ti-Pack 110 is positioned inside a cavity162 within a steel die 112. However, to improve the structuralefficiency of the SPF/DB hollow core concept, a metal matrix skin designis incorporated. To do this, two individual composite fiber mats 184,186, preferably silicon carbide fiber mats, are added between the facesheet 154 and the outer core sheet sections 130 during the Ti-Packbuild-up process, as shown in FIG. 21. The fibers run in thelongitudinal direction and provide increased bending stiffness. Thewidth of the fiber mats 184, 186 are cut so that they overlap oneanother at the leading edge 188. As the face sheet 154 is formed out tothe tool surface 168, the fiber mats will start to uncurl as indicatedin FIG. 22. The mats are then pushed out (strain-free) to the face sheet154 by the outer core sheet section 130 as it begins to form. FIG. 23shows the inner and outer core sheets 128 and 130, respectively, as theyexpand outward. The gas pressure is increased at this point to promotethe consolidation of the fibers in the mats 184, 186. Over time andelevated temperature, the sheets will form a titanium metal matrixstructure. The final blade configuration, shown in FIG. 24, looksidentical to that of the first embodiment (FIG. 19) except that it willcontain the silicon carbide fibers between the face sheet and the outercore sheet section.

FIGS. 25 and 26 show an alternative blade assembly 178 to that shown inFIG. 20, with a root section 180 which is more closely adapted to theSPF/DB hollow core concept. In the embodiment of FIG. 20, the rootsection 80 is a known solid root section, currently state of the art forsolid fan blades. Thus, the hollow blade body 76 is welded to a solidroot section 80. A much better approach, however, is to flare out theend of the rotor blade to form the root section 180, the hollow portionthen being filled with a solid material filler such as a pottingcompound (epoxy filler) 190, as shown in FIG. 26. A titanium slug or thelike could also be used as the solid material filler. The root section180 and the blade body 176 then become a homogeneous member, thusrequiring no welding and providing an excellent means of load transferthrough the root section into the blade retainer hub.

Although exemplary embodiments of the invention have been shown anddescribed, many changes, modifications, and substitutions may be made byone having ordinary skill in the art without departing from the spiritand scope of the invention. Therefore, the scope of the invention is tobe limited only in accordance with the following claims.

What is claimed is:
 1. A hollow core rotor blade for a turbine engine,comprising:a generally airfoil-shaped outer structure comprised of asuperplastically formed, diffusion bonded sheet material, said outerstructure having a trailing edge and a leading edge, said leading edgehaving an outer surface; and a hollow core spacing having at least oneweb extending therethrough and being enclosed by said outer structure,said at least one web being either generally vertical or generallyhorizontal in orientation; wherein the outer surface of said leadingedge is formed from a single sheet of material and is thereforestructurally continuous and seamless, thereby allowing said rotor bladeto be relatively lightweight, efficient, and durable.
 2. The rotor bladeas recited in claim 1, wherein said at least one web comprises aplurality of vertical or horizontal webs.
 3. The rotor blade as recitedin claim 1, wherein said at least one web comprises a plurality ofvertical and horizontal webs.
 4. The rotor blade as recited in claim 1,wherein said turbine engine is a gas turbine engine and said rotor bladeis a fan blade.
 5. The rotor blade as recited in claim 1, wherein saidouter structure is comprised of a titanium alloy.
 6. The rotor blade asrecited in claim 1, wherein said outer structure is comprised of amatrix structure, with composite fibers being embedded within saidsuperplastically formed material.
 7. The rotor blade as recited in claim6, wherein said fibers are oriented in a longitudinal direction, therebyincreasing bending stiffness of the blade.
 8. The rotor blade as recitedin claim 6, wherein said fibers comprise silicon carbide.
 9. The rotorblade as recited in claim 6, wherein said outer structure is comprisedof a titanium metal matrix structure, with silicon carbide fibers beingembedded therein, said silicon carbide fibers being oriented in alongitudinal direction to increase bending stiffness of the blade. 10.The rotor blade as recited in claim 6, wherein said composite fiberscomprise silicon carbide fiber mats.
 11. A hollow core rotor blade for aturbine engine, comprising:a generally airfoil-shaped outer structurecomprised of a superplastically formed, diffusion bonded sheet material,said outer structure having a trailing edge and a leading edge and beingcomprised of a matrix structure, with generally longitudinally orientedcomposite fibers being embedded within said superplastically formedmaterial to increase the bending stiffness of the blade, said leadingedge having an outer surface; and a hollow core spacing enclosed by saidouter structure; wherein the outer surface of said leading edge isformed from a single sheet of material and is therefore structurallycontinuous and seamless, thereby allowing said rotor blade to berelatively lightweight, efficient, and durable.
 12. The rotor blade asrecited in claim 11, wherein said fibers comprise silicon carbide. 13.The rotor blade as recited in claim 11, wherein said composite fiberscomprise silicon carbide fiber mats.
 14. The rotor blade as recited inclaim 11, wherein said matrix structure comprises titanium.